1. Field of the Invention
The present invention relates to a method for producing hematite for ironmaking.
2. Description of the Related Art
More particularly, the present invention relates to a technique for separating a tailings slurry obtained after a nickel oxide ore is treated though a final neutralization step according to a high pressure acid leach (HPAL) process in a hydrometallurgical refining plant, thereby recovering hematite for ironmaking.
Nickel has been widely used as a raw material of stainless steel, but in accordance with the tendency of depletion of a sulfide ore that becomes a raw material of nickel, a technology of refining a low-grade oxide ore has been developed and has been into practical use.
Specifically, a producing process called a “high pressure acid leach (HPAL)” process has been put into practical use, in which process a nickel oxide ore such as limonite and saprolite is put into a pressurizing apparatus such as an autoclave in combination with a sulfuric acid solution, and nickel is leached under a high temperature of approximately 240° C. to 300° C. and a high pressure. FIG. 3 illustrates a schematic flowchart of the producing process.
A neutralizing agent is added to nickel that is leached in the sulfuric acid solution by the HPAL process to neutralize a surplus acid, and then nickel is subjected to solid-liquid separation treatment to be separated from a leach residue.
Then, the nickel is recovered as an intermediate raw material in a form of hydroxide or sulfide through a process of separating impurities, and the intermediate raw material is further refined to obtain nickel as nickel metal, a nickel salt compound, and the like.
In the neutralization process of neutralizing the surplus acid, pH of the leach substance is adjusted to be appropriate for the solid-liquid separation, and then in a subsequent process or the solid-liquid separation process concentration of a solid content and solid-liquid separation are performed in a facility called counter current decantation (CCD). Typically, in CCD, a plurality of continuous stages of thickeners are used.
A liquid component (hereinafter, may be referred to as an overflow), which is obtained from the CCD, is returned to a neutralization process for adjustment of pH that is appropriate for a sulfurization process. In the neutralization process, a pH adjustment is performed to remove a fine solid content that occurs, through precipitation. Then, for example, the liquid component is subjected to a sulfurization treatment, whereby an intermediate raw material such as a nickel sulfide is obtained.
Employing the HPAL process makes it possible to leach nickel almost completely, for example, in the case of nickel oxide ore, even in a low grade ore in which a target valuable metal to be recovered is contained in an amount of 1% by weight to 2% by weight (hereinafter, the weight will be expressed by “%”). Accordingly, a target metal is concentrated to the same extent as in a conventional raw material, and the target metal can be obtained through substantially the same refining method and refining process as in the conventional raw material. In addition, the HPAL process is applicable to not only the nickel oxide ore but also different raw materials, such as a nickel sulfide ore, a copper sulfide ore, and a copper oxide ore.
Besides, a main component of the leach residue that is obtained by the HPAL process is an iron oxide, and approximately 40 to 50% of iron is contained in the solid content of the leach residue. In addition, Production amount of the leach residue is approximately 50 times to 100 times as much as that of the intermediate raw material. The reason for this is that each of the nickel oxide ore or the sulfide ore that is a raw material contains iron in an amount much more than that of nickel.
The leach residue is generated at a high temperature, and is in a type of a chemically and environmentally stable oxide, but has no particular utility value in a current state, and has been thus scrapped and stored in a residue disposal yard.
Therefore, a broad residue disposal yard is necessary for scrap and storage of an enormous amount of the leach reside which is generated in accordance with the HPAL process operation.
By the way, many of the iron oxide is contained in an iron ore and the iron ore has been widely used as a raw material for refined steel.
In steel smelting, target steel is produced by charging iron ore containing iron oxide into a blast furnace along with a reductant such as coke, heating and melting the iron ore under a reducing atmosphere to obtain crude steel, and then refining the crude steel in a converter.
Typically, the iron ore as the raw material is a limited resource, and availability of an iron ore with high quality that is necessary for quality maintenance of steel becomes gradually difficult. Accordingly, a study has been made with respect to use of the leach residue as the iron ore.
However, the leach residue in the HPAL process cannot be directly used for a raw material for ironmaking for the following reasons.
The leach residue in the HPAL process contains vein stone or impurities, particularly sulfur, in addition to the iron oxide, and thus the leach residue is not appropriate for a raw material that is used in a conventional ironmaking process in common. Specifically, this is because the sulfur content is high.
Particularly, the sulfur content in the iron oxide which can be used for a raw material for ironmaking is different depending on facility capacity, an amount of production, and the like in individual ironworks. However, typically, it is necessary to suppress the sulfur content to less than 1%.
On the other hand, the solid-content leach residue contains approximately 5% to 8% of sulfur. The majority of sulfur in the leach residue is derived from calcium sulfate (gypsum) that is mixed in during the HPAL process.
When neutralizing free sulfuric acid (the free sulfuric acid is sulfuric acid that remains without reaction in the sulfuric acid that is excessively added for performing sufficient leaching in the HPAL process), which remains in leach slurry that is obtained during high-pressure acid leaching, a typical inexpensive calcium-based neutralizing agent, for example, limestone or slaked lime is added. Accordingly, when calcium contained in the neutralizing agent and the free sulfuric acid react with each other, the gypsum is generated and is then mixed into the leach residue.
A part (approximately 1%) of sulfur that is contained in the solid-content leach residue is trapped inside particles of hematite that is generated.
The solid content in the residue after nickel leach obtained at this point is mainly constituted by hematite of approximately 1 μm, and the solid content contains the iron grade of approximately 30 to 40% and the sulfur grade of approximately 5 to 8%. The moisture content of the leach residue obtained at this point is 60%.
In order to use the leach residue as hematite for ironmaking, it is necessary to achieve the iron grade of 50% or more and the sulfur grade of 1% or less in the solid-content leach residue.
To achieve this, for example, JP 2010-095788 A discloses a technique for removing impurities contained in a mixture of hematite by separating a leach residue by sieve classification, wet-cyclone separation, and magnetic separation, and the technique has been recognized to have some effect on removal of impurities contained in hematite.
However, the method is not enough to use the leach residue after the physical separation alone as hematite for ironmaking as described above, and in particular, the iron content only increases to approximately 40 to 45% at most. Accordingly, in order to use such a leach residue as hematite for ironmaking, it was necessary to mix with a raw material for ironmaking which contains a higher grade of iron. The moisture content of the leach residue obtained by physical separation is approximately 40%.
The present invention has been made in consideration of these problems to propose a technique for separating a leach residue from which a hematite-containing material capable of being used as a raw material for ironmaking can be obtained and to provide a method for producing hematite for ironmaking from the leach residue.